1. Field of the Invention
The invention relates to a method for fabricating a complimentary metal-oxide semiconductor (CMOS) transistor.
2. Description of the Prior Art
More recently, dual liner techniques have been introduced in order to provide different stresses in P-type MOS devices with respect to N-type MOS devices. For example, a nitride liner of a first type is formed over PMOS transistor of a CMOS device, while a nitride liner of a second type is formed over the NMOS transistor of the CMOS device. More specifically, it has been discovered that the application of a compressive stress in a PMOS channel improves carrier mobility therein, while the application of a tensile stress in an NMOS channel improves carrier mobility therein. Thus, the first type nitride liner over the PMOS devices is formed in a manner so as to achieve a compressive stress, while the second type nitride liner over the NMOS devices is formed in a manner so as to achieve a tensile stress.
For such CMOS devices employing dual liners, the conventional approach has been to form the two different nitrides using separate lithographic patterning steps. In other words, for example, the first type nitride liner is formed over both PMOS and NMOS devices, with the portions of the first type nitride liner over the NMOS devices being thereafter patterned and removed. After an optional formation of an oxide layer, the second type nitride liner is formed over both regions, with a second patterning step being used to subsequently remove the portions of the second type nitride liner over the PMOS devices. Unfortunately, due to inherent inaccuracies associated with aligning lithographic levels to previous levels, the formation of the two liners could result in an underlap therebetween.
In particular, the two liners could be formed in a manner such that one liner overlaps the other in a boundary region between the NMOS region and the PMOS region. In fact, the reticles used for the two separate patterning steps are typically designed to ensure an overlap such that there is no gap between the two liner materials.